1. Field of the Invention
The present invention relates to an epitaxial semiconductor wafer manufacturing method, and particularly to an epitaxial semiconductor wafer manufacturing method of offsetting a warpage of a wafer produced due to epitaxial treatment by an original warpage of the wafer to reduce the warpage after the epitaxial treatment.
2. Description of the Prior Art
A semiconductor wafer made of silicon or the like used as a material of a semiconductor device is manufactured by passing through various steps such as a slicing step of cutting off a wafer from an ingot, a grinding step of flattening the wafer and a polishing step of mirror-finishing the surface of the wafer. By further applying epitaxial treatment to the surface of the semiconductor wafer thus manufactured, an epitaxial semiconductor wafer is manufactured.
One of the problems in manufacturing an epitaxial semiconductor wafer is the warpage of the wafer. That is, by applying epitaxial treatment to the surface of a wafer, a warpage of approx. tens of microns which becomes convex in a direction of the wafer surface with an epitaxial layer grown on it is produced because of a stress difference between a surface and a back of the wafer due to the epitaxial layer grown on the wafer surface.
To solve the above problem, Japanese Patent Laid-Open No. 6-112120 specification discloses a method of reducing the warpage of a wafer after epitaxial treatment by measuring the warpage before the epitaxial treatment is executed, adjusting the direction of the warpage to the direction opposite to the direction in which the wafer is warped and thereby offsetting the former warpage by the latter warpage produced due to the epitaxial treatment.
It is considered that the direction of the warpage of a wafer before epitaxial treatment is almost decided in a slicing step of slicing an ingot to a wafer and the initial warpage direction is kept even after passing through the grinding step and polishing step after the slicing step. Therefore, by designating the surface and back for slicing and executing subsequent steps, a warpage after epitaxial treatment is reduced.
A wafer cut out from an ingot by a wire saw has a large unevenness of approx. tens of microns. It is one of large objects of a grinding step to remove the unevenness from a wafer and flatten the wafer. To remove the unevenness, both-side simultaneous grinding is necessary. From this viewpoint, a double-sided lapping machine has been frequently used so far.
However, wafers have been recently increased in diameter and a double-sided lapping machine used for a large-diameter wafer has the following problems.
(1) Increase of costs of consumable materials and machine due to an increased size of the machine
(2) Increase of wafers in diameter and increase of operator in load due to increase of wafers in diameter
(3) Increase of industrial waste (waste polishing powder) due to increase of wafers in diameter
Therefore, it is recently proposed to manufacture a wafer by a double-sided grinding machine instead of a double-sided lapping machine. The double-sided grinding machine simultaneously grinds the both sides of a wafer by symmetrically holding a part of the wafer from its both sides by a cup-shaped grindstone. Therefore, the double-sided grinding machine is not only capable of removing the unevenness of a wafer described above but also effective for solving the above problems (1) to (3) which are also problems of a large lapping machine.
However, it is clarified from the examination by the present inventor that when using a double-sided grinding machine instead of a double-sided lapping machine, the shape of the warpage of a wafer which is considered to be decided in a slicing step and left as it is greatly changed due to grinding of the wafer and the direction of the warpage is even inverted in an extreme case. Moreover, it is clarified that a warpage deformed due to the machining by a double-sided grinding machine is not stably kept to the end but the deformed warpage is comparatively easily returned to the true shape by removing the surface layer of the wafer and the stress remaining in the wafer.
Thus, an unstable warpage is created by performing machining by a double-sided grinding machine. Therefore, when measuring the direction of the warpage of a wafer before machining it by a double-sided grinding machine and measuring the direction of the warpage of the wafer immediately after machining the wafer, the measured warpage direction may be different from the actual warpage direction in epitaxial treatment and as a result, an epitaxial layer is formed on a face opposite to the face on which the warpage can be offset by epitaxial treatment, and the warpage produced by epitaxial treatment cannot be offset but the warpage may be accelerated.
It is an object of the present invention to provide an epitaxial semiconductor wafer manufacturing method capable of securely offsetting a warpage through epitaxial treatment even if performing effective grinding using a double-sided grinding machine.
The following is the reason why a new warpage is created through machining by a double-sided grinding machine. In the case of a double-sided lapping machine, the whole wafer is held by a surface plate having an area larger than that of a wafer and the entire surface of the wafer is simultaneously machined. The wafer currently machined is fixed in the thickness direction and a new warpage due to the thickness-directional movement of the wafer is not formed. In the case of a double-sided grinding machine, however, because a wafer is held in line contact by grindstones having an area smaller than that of the wafer, a force of constraint for fixing the wafer in its thickness direction while being machined only works on a part of the wafer held by the grindstones.
That is, because a wafer while being machined by a double-sided grinding machine is not sufficiently fixed in the thickness direction, the wafer may move in its thickness direction. Therefore, the movement of the wafer in its thickness direction forms a warpage after the wafer is machined.
An epitaxial semiconductor wafer manufacturing method of the present invention makes it possible to securely offset a warpage in epitaxial treatment by measuring the direction of the warpage of a semiconductor wafer, flattening the semiconductor wafer by a double-sided grinding machine when adjusting the direction of the warpage and performing epitaxial treatment, removing a machining-strain layer formed on the both sides of the semiconductor wafer due to the above flattening, and measuring the direction of the warpage of the wafer.
That is, by machining a wafer cut by a wire saw by a double-sided grinding machine, a strain layer formed on the surface of the wafer when it is cut and an unevenness component are removed and the flatness of the wafer is improved. However, a new warpage is formed due to the unbalance between residual strains of the surface and back of the wafer and the wafer is changed to a shape different from the warpage shape formed in the wafer cutting process. The direction of the warpage of the wafer measured under the above state is unstable. However, by removing the residual-strain layer formed due to the machining by the double-sided grinding machine, the warpage returns to the true shape and by measuring and adjusting the direction of the warpage under this state, directions of warpages of all wafers are adjusted to the direction for offsetting a warpage formed due to epitaxial treatment.
It is sufficient to execute a step of removing a machining-strain layer before performing epitaxial treatment after performing machining by a double-sided grinding machine. However, when executing the step after polishing for mirror-finishing, the mirror-finishing may be necessary again. Therefore, it is preferable to execute the step before polishing. Moreover, it is preferable to execute a step of measuring and adjusting the direction of a warpage before polishing in order to avoid attachment of particles. That is, it is the most rational process to remove a machining strain and adjust the direction of a warpage, and then perform mirror-polishing and epitaxial treatment. By performing etching before the mirror-polishing, it is possible to remove a new strain due to removal of a machining-strain layer and reduce a polishing amount in mirror-polishing.
Etching is considered as a method of removing a machining strain. When only etching is performed, however, the flatness (parallelism) is reduced due to the etching and a load increases due to the subsequent polishing. A preferable machining-strain removal method is a method of removing a surface machining-strain layer by an inverting single-sided grinding machine and thereby, the flatness (parallelism) of a wafer can be maintained. That is, because the inverting single-sided grinding machine grinds one side of a wafer while attracting and fixing the other side of the wafer to a surface plate, it is possible to fix the wafer in its thickness direction and remove a machining-strain layer without causing a warpage due to the difference between stresses of the surface and back of the wafer. However, because of the single-sided grinding machine, it is difficult to remove an unevenness produced due to cutting. Moreover, using alkali cleaning or slight etching together for inverting single-sided grinding is preferable because it is possible to reduce a polishing amount in the next step.
By measuring the true warpage direction of a semiconductor wafer and adjusting the direction and then executing marking, it is possible to put a mark on the same face to the warpage direction and determine the direction of the warpage in accordance with the mark.